10_macros.rkt

#lang racket ; defines the language we are using

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;; 10. Macros
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;; Macros let you extend the syntax of the language

;; Let's add a while loop
(define-syntax-rule (while condition body ...)
  (let loop ()
    (when condition
      body ...
      (loop))))

(let ([i 0])
  (while (< i  10)
    (displayln i)
    (set! i (add1 i))))

;; Macros are hygienic, you cannot clobber existing variables!
(define-syntax-rule (swap! x y) ; -! is idiomatic for mutation
  (let ([tmp x])
    (set! x y)
    (set! y tmp)))

(define tmp 2)
(define other 3)
(swap! tmp other)
(printf "tmp = ~a; other = ~a\n" tmp other)
;; The variable `tmp` is renamed to `tmp_1`
;; in order to avoid name conflict
;; (let ([tmp_1 tmp])
;;   (set! tmp other)
;;   (set! other tmp_1))

;; But they are still code transformations, for example:
(define-syntax-rule (bad-while condition body ...)
  (when condition
    body ...
    (bad-while condition body ...)))
;; this macro is broken: it generates infinite code, if you try to use
;; it, the compiler will get in an infinite loop